1. Technical Field
The present disclosure relates to an image sensor and a method of driving the same and, more particularly, to a CMOS image sensor (CIS) type image sensor using a single-slope analog-to-digital converter (ADC) and a method of driving the same.
2. Discussion of Related Art
CMOS image sensors are commonly used in portable cameras, digital still cameras, web cameras and the like to convert an image into a digital signal. The digital image signal that is output from the CMOS image sensor may include various color signals. The digital image signal is processed to drive a display device such as a liquid crystal display.
The CMOS image sensor uses a correlated double sampling (CDS) method and uses a ramp signal to generate a digital signal from the difference between a reset signal and an image signal which are sampled according to the CDS method. That is, the CMOS image sensor picks up the difference between the reset signal and the image signal that varies based on external light illumination, and generates a digital code corresponding to the difference. The digital code varies depending on the slope of the ramp signal under the same illumination conditions. To maintain the same luminosity or brightness under the same illumination conditions, the ramp signal has to be constant when the image acquired by the CMOS image sensor is displayed on a display device.
An analog built-in self calibration (BISC) scheme has been conventionally used for a design for testability (DFT), and may be used to constantly supply the ramp signal that significantly affects image quality. In the conventional analog BISC scheme, the ramp signal is adaptively calibrated to coincide with an analog target voltage using an analog comparator. Variations in manufacturing processes, analog target voltages, and offsets of an amplifier used for a ramp signal calibration can be calibrated to some extent by the conventional scheme. However, the ramp signal continuously fluctuates around the target voltage due to frequency changes in a clock signal used in a ramp signal calibration, a ramp signal noise and the like, which causes deterioration of the signal-to-noise ratio (SNR) of the digital image signal output from the CMOS image sensor and limits BISC operation.